This is my take on what the sections of code in lecture 20 mean, based on what I have learned so far.
Preprocessor Directive: Indicating that premade code should be uploaded before the compiling of the actual code.
Main Function: Creates the body of the program, where all the code, whether directly or through a function, is run.
Expression Statements: Any statement that ends with a semicolon and is not a declaration statement.
Declaration Statements: A statement that declares something. (usually a variable)
Return Statement: Returns a value and ends the function.
Please tell me if I have misinterpreted any of these! Any feedback would be much appreciated. 
I’m also not sure if I’m posting if in the right place? I’m new here, so please also tell me if I have misplaced this post!